OPINIONS: The Gemara discusses various situations in which Sechach was placed upon the Sukah in such a way that the Sukah is rendered invalid (for example, the Sechach was attached to the ground (Mechubar), or it was not placed upon the Sukah for the sake of providing shade (l'Shem Tzel; see Rashi 12a, DH Chada). The Gemara teaches that in such cases, in order to make the Sukah valid it does not suffice merely to cut the Sechach from the ground (in a case of Mechubar) or to have intention that, from now on, the Sechach should be for the sake of shade (when it was placed there for a purpose other than for shade). Rather, one must remove all of the pieces of the Sechach and place them back on the Sukah with the proper intent. The requirement for active placement, with proper intent, of valid Sechach is called, "Ta'aseh v'Lo Min he'Asuy," and it is cited as the Halachah by the SHULCHAN ARUCH (OC 626:2).
What is the Halachah in a case of a Sukah that is invalid not because of an inherent factor in the Sechach, but because of external factors? For example, when one corrects a Sukah that was taller than 20 Amos, or that was built underneath a tree or inside of a house (he raises the floor of the tall Sukah such that its Sechach is within 20 Amos from its floor, or he removes the tree or roof which covered the Sukah), does he need to lift up the Sechach and return it in order for the Sukah to be valid?
(a) The HAGAHOS ASHIRI (1:23) cites RABEINU BARUCH of Regensburg who asserts that the rule of "Ta'aseh v'Lo Min he'Asuy" applies in such cases, and one must lift up the Sechach in order to validate the Sukah.
(b) However, RABEINU YITZCHAK HA'LAVAN (cited by the Hagahos Ashiri), the KOL BO (cited by the Darchei Moshe, beginning of OC 626), and RABEINU ELCHANAN (cited by the Beis Yosef, end of OC 626) rule that it is not necessary to do anything to the Sechach in these cases. The Sukah becomes valid as soon as the height is lessened or the tree removed. They reason that the only situation in which Sechach must actively be lifted up and returned is when the Pesul is inherent in the Sechach itself (such as Sechach that was Mechubar, or that was not placed there l'Shem Tzel). If the Pesul is not inherent in the Sechach but is in some other feature of the Sukah, then it suffices to correct that other feature. The Kol Bo adds that since the Pesul is not in the Sechach, the corrective act of "Ta'aseh" (the significant act that is required when one makes a Sukah) needs to be done only in the part of the Sukah that is invalid.
This opinion is cited as the Halachah by the Shulchan Aruch (OC 626:2-3, and Mishnah Berurah there).
The Halachah mentioned above involves a case of a Sukah built inside of a house. The Acharonim discuss a third type of case -- Sechach that is put into place underneath a removable roof (one that can be rolled up or lifted up with a crank or other mechanism). In such a case, after the roof is removed from above the Sechach, must the Sechach be completely re-laid in order for the Sukah to be valid?
The MAHARIL (cited by the BACH, end of OC 626) writes that the Halachah in this case is the same as the Halachah in the case of a Sukah built inside of a house. Once the invalidating factor is corrected, no other action is necessary. Nevertheless, the Maharil quotes a "Ga'on Echad" who says that in this case one must lift up each piece of Sechach and re-lay it. The Bach cites the MAHARSHAL who sides with this ruling.
However, it is not clear why the Ga'on Echad and Maharshal differentiate between this case and the previous case.
The Bach suggests that the Ga'on is stringent in this case because the Sechach cannot be called "inherently valid Sechach" before the roof is removed, because the Sechach is not part of a complete, independent Sukah. Placing branches beneath a roof does not make the room a Sukah. This case cannot be compared to a complete Sukah with its own walls and Sechach, built underneath a tree or inside of a house, because the invalidating factor is, to a certain degree, inherent in the Sechach itself.
The MAGEN AVRAHAM offers a different explanation. He explains that lifting up a removable roof by pulling it up with a rope is not considered a significant action. As mentioned earlier (see (b) above), the Kol Bo maintains that even if the invalidating factor is not in the Sechach itself, one must perform a significant act to make the Sukah valid. The lifting of such a roof does not constitute a significant act.
HALACHAH: In the case of Sechach laid under a lift-up roof, the MISHNAH BERURAH (OC 626:18) requires that the Sechach be lifted up and returned to its place after the roof is moved.
This requirement applies, however, only when the Sechach was put into place while the roof was still in position. If the Sechach was put into place after the roof was removed (and the Sukah, at that moment, was valid), then even if the roof is later lowered back onto the Sechach (for example, to protect the Sukah from the rain), it suffices merely to lift off the roof again in order for the Sukah to be valid (Mishnah Berurah 626:19).


QUESTION: Rebbi Eliezer teaches that the Sukah commemorates the Ananei ha'Kavod, the Clouds of Glory, that surrounded the Jewish people during their sojourn in the desert. Rebbi Akiva teaches that the Sukah commemorates the actual Sukos in which the Jews lived in the desert, and which protected them from the desert elements.
From where did the Jews obtain wood in the desert for building Sukos?
(a) RABEINU BACHYE (Vayikra 23:43) comments that they purchased wood from merchants of other nations (see Yoma 75b, where the Gemara teaches that the Jews bought food from foreign merchants). The miracle that the Sukah commemorates is that for forty years Hash-m provided the Jews with their every need as they wandered through an inhospitable and uninhabitable desert.
(b) The MIDRASH TANCHUMA (Parashas Chukas) says that the waters of Miriam's well flowed around the encampment and caused a variety of trees and hedges to sprout there. Perhaps it was the wood from those trees that the people used to build their Sukos. The miracle that the Sukah commemorates is that Hash-m miraculously provided trees for the Jews in the desert.